A clue to the function of intramembranous particle assemblies in astrocytes may be gained by correlating their changes with those in the cytoplasm of reactive glial cells. The cytoplasmic change being followed, immunocytochemically, is that in glial fibrillary acidic protein (GFAP). The glia limitans in 2 week old rats provides a good baseline because it has little or no detectable GFAP at that age. Changes in GFAP reactivity were examined from 3 hours to 2 weeks after a localized freezing lesion was made to the cerebral cortex of 2 week old rats. The earliest staining of GFAP appeared by 24 hours, about a day earlier than that reported for a stab wound. The GFAP response occurred in the astrocytes at the periphery of the cold lesion where the assembly numbers increased. However, the increment in assemblies was considerably more rapid: 30 minutes to 4 hours after the lesion was made. Thus, the assemblies which appear to be physically linked with actin filaments and microtubules, do not appear to be directly associated with intermediate filaments; the addition and distribution of new assemblies is unrelated to the presence of GFAP. Two changes took place within the cell membrane of another cell type: arachnoid. At the periphery of the same lesion, the arachnoid response was about as rapid as the assembly increment. Within the first 3 hours after the lesion was made, there was a pronounced increase in the linear extent and number of ridges or strands belonging to tight junctions of subdural arachnoid cells. A greater number of gap junctions formed between tight junction strands between deeply situated, reactive cells than in normal, resting arachnoid cells. In some of the arachnoid cell membranes there were short, discontinuous strands, suggestive of new, forming junctions. In both glial and meningeal reactive cells, the intramembranous responses preceded the cytoplasmic change in the glial cells.